Electrode structure for capacitance-type measurement transducers

ABSTRACT

A capacitance-type measuring apparatus includes first and second support members which are movable with respect to one another along a measurement axis; a first electrode array and a second electrode disposed on the first support member; and a third electrode array disposed on the second support member and capacitively coupling the first electrode array and second electrode for transmission of a signal between the first electrode array and second electrode through the third electrode array. One of the first electrode array or the second electrode is formed to have end portions configured such that the degree of coupling between the first electrode array and second electrode through the third electrode array decreases linearly along the extent of each end portion relative to the measurement axis. This configuration permits improved measurement accuracies by reducing the effects of tilt between the first and second support members which changes the spacing between such members along the axis of measurement.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part application ofcopending U.S. application Ser. No. 07/200,368 filed May 31, 1988, nowU.S. Pat. No. 4,879,508, which in turn is a continuation-in-part of U.S.application Ser. Nos. 07/030,346 and 07/031,049, both filed Mar. 26,1987, and Ser. No. 07/035,859, filed Apr. 8, 1987 all of which are nowabandoned.

FIELD OF THE INVENTION

The present invention relates generally to measuring devices for makinglinear and angular measurements, and more particularly to acapacitance-type measuring transducer with an improved electrodeconfiguration for making measurements of position.

BACKGROUND OF THE INVENTION

Numerous capacitance-type measuring devices for making linear andangular measurements have been developed wherein two support members orscales, on which are respectively mounted arrays of discrete,capacitively coupled electrodes, are displaced relative to one another,and the relative positions of the two scales are determined by sensingthe resulting change in the capacitance pattern created by the arrays ofelectrodes. Typically, the capacitance pattern is sensed by applying aplurality of periodic signals to one of the electrode arrays andmeasuring the shift in signals resulting from the transfer to the otherarray of electrodes. Such measuring devices have a broad range ofapplications, from large-scale measuring devices such asthree-dimensional coordinate measuring systems and numericallycontrolled finishing machines, to small-scale devices such as portablecalipers, micrometers and the like.

In addition to the increasing popularity of the capacitance-typemeasuring devices, a wide variety of configurations and designs bavebeen proposed which implement both relative and absolute measurements.While various improvements have been made to increase the capabilitiesof such capacitance-type measuring devices, there are stilldisadvantages which limit the accuracy with which measurements can bemade or increase the cost of constructing the measuring devices in orderto compensate for the accuracies. In particular, many applicationsrequire the ability to obtain low-cost accurate measurements even whenthe devices will be subjected to hostile conditions in the environmentsin which they are used. As a result, if the devices are extremelysensitive during use, then their use will be limited by the ability tocontrol the application environment or the need to obtain accuracy byexpensive designs or manufacturing techniques.

More specifically, in the use of capacitance-type measuring devices ofthe type described having two scales relatively movable with respect toone another, the gap between the scales should be uniform over theentire area of overlap. Experience and theoretical calculations showthat parallelism in the x/y-plane is the most important factor affectingaccuracy. This sensitivity to tilt (i.e., rotation of one scale relativeto the other around an axis perpendicular to the intended measurementdirection in the plane of the scales) limits the achievable accuracy ata given mechanical tolerance in the suspension system guiding themovement of the two scales relative to each other. Or, for a givenaccuracy specification, it may require a tolerance level on themechanical design that is not practical or is too expensive toimplement.

The present invention, therefore, has been developed to overcome thespecific shortcomings in the implementation of capacitance-typemeasuring devices and to provide an electrode configuration whichimproves measurement accuracy.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a transducercomprises first and second support members, the support members beingrelatively displaceable with respect to each other and relative to ameasurement axis; first electrode structure disposed on the firstsupport member and second electrode structure disposed on the secondsupport member and capacitively coupled to the first electrodestructure; the first and second electrode structures each having anelongate configuration relative to the measurement axis, and one of thefirst and second electrode structures having at least one end portionconfigured such that the degree of coupling between the first and secondelectrode structures decreases linearly relative to the measurement axisalong the extent of each end portion.

In accordance with another embodiment of the present invention, thetransducer includes first and second electrode structures disposed onthe first support member; a third electrode structure disposed on thesecond support member and capacitively coupled to the first and secondelectrode structures for transmission of a signal between the first andsecond electrode structures through the third electrode structure; andthe first and second electrode electrode structures are constructed toeach have an elongate configuration relative to the measurement axis andsuch that one of the first and second electrode structures has at leastone end portion configured such that the degree of coupling between thefirst and second electrode structures through the third electrodestructure decreases linearly relative to the measurement axis along theextent of each end portion.

Advantageously, the transducer comprises an array of first electrodesdisposed on the first support member in alignment with the measurementaxis; an array of third electrodes disposed on the second support memberin alignment with the measurement axis and such that different portionsof the third electrode array are capacitively coupled with the firstelectrode array in dependence on the relative positions of thesupporting members; and a second electrode disposed on the first supportmember in relative alignment with the first electrode array andcapacitively coupled with the third electrode array in dependence on therelative position of the supporting members, one of the first electrodearray and the second electrode being constructed to have the at leastone end portion.

In accordance with a further aspect of the present invention, the firstelectrodes are arranged so as to constitute successive groups of apredetermined number of adjacent electrodes, the groups each having awavelength Lt; and the second electrode has a length which is less thanthe length of the first electrode array and is substantially equal to aninteger number of wavelengths Lt. Advantageously, the second electrodeis configured so that the opposite ends thereof terminate in a taperedconfiguration. The second electrode is also advantageously configured sothat the effective width of each end portion tapers from a full width toterminate at the respective ends of the electrode over a distance thatis substantially equal to an integer number of wavelengths Lt.

In accordance with a further aspect of the present invention, the arrayof first electrodes is formed to define an envelope having an elongateconfiguration with a length which is less than the length of the secondelectrode and substantially equal to an integer number of wavelengthsLt. The first electrode array envelope advantageously further includesend portions in each of which the lengths and/or the widths of theindividual electrodes progressively taper from a full length/width totermination at the respective ends of the array over a distancesubstantially equal to an integer number of wavelengths Lt.

These and other novel features and advantages of the present inventionare described in or will become apparent from the following detaileddescription of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments will be described with reference to thedrawing, wherein like elements have been denoted throughout the figureswith like reference numerals, and wherein:

FIG. 1 is a partly schematic, partly diagrammatic view of acapacitance-type measuring device.

FIG. 2A is a diagrammatic view showing the possible axis of movement oftwo support members in a capacitance-type measuring device positioned ina three-dimensional coordinate system.

FIGS. 2B-2F are more detailed schematic depictions of the relativemovements shown in FIG. 2A.

FIG. 3 is a diagrammatic view of a capacitance-type measuring deviceincluding a first embodiment of a transducer incorporating a taperedelectrode structure in accordance with the present invention.

FIG. 4 is a diagrammatic view of a capacitance-type measuring deviceincluding a second embodiment of a transducer incorporating a taperedelectrode structure in accordance with the present invention.

FIGS. 5-8 are plan views of alternative embodiments of tapered electrodestructures according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in connection with itsapplication to a linear measurement device. It will be understood,however, that the present invention is not limited to such measuringdevices and may be implemented in a broad range of other devices formaking a variety of measurements. Referring first to FIG. 1, there isshown a capacitance-type linear measurement caliper 10. The constructionand configuration of the caliper 10 may be any of a variety of forms,several examples of which are described more fully in applicant'scopending U.S. patent applications Ser. Nos. 07/200,368 and 07/200,580,both filed May 31, 1988, and herein incorporated by reference in theirentirety.

Briefly, the caliper 10 includes a capacitive transducer 12 andelectronic measuring apparatus 100 comprising a signal generator 102 andsignal processor 104 for applying electrical excitation signals totransducer 12 and processing the resultant output signals produced bytransducer 12 to ascertain a given measurement position. Transducer 12comprises a linear first scale or support member 20, and a shorterlinear second scale or support member 30, slidably mounted with respectto member 20 for longitudinal axial displacement along a predeterminedmeasurement axis X. Support member 30 is generally known as the pick-offmember and each of the members 20 and 30 are generally provided withextending caliper arms (not shown) which are used to permit measurementsof the dimensions of an object.

The support members 20 and 30 each have a planar face on which aredisposed various electrode arrangements, or structures 210, 220 and 310,320, respectively, as will be more fully described below. Supportmembers 20 and 30 are spaced in opposed, parallel relation to oneanother and slidably supported so that the support member 30 may bedisplaced longitudinally along the measurement axis with respect to themember 20. By way of example, the distance d separating the supportmembers 20 and 30 is advantageously on the order of 0.05 mm (0.002inch).

As described below and more particularly in the aforementioned copendingapplications, the electrode structures 310 and 320 are disposed adjacentone another on support 30 in relative alignment with the measurementaxis X. Likewise, electrode structures 210 and 220 are electricallyconnected and disposed adjacent to one another on support member 20 inrelative alignment with the measurement axis. As shown, the excitationsignal outputs of signal generator 102 are connected to the electrodesof structure 310 and signal processor 104 is connected to electrodestructure 320 to receive the output of transducer 12. Accordingly, forconvenience of description, the electrodes of structure 310 (hereinafterarray 310) will be described as first transmitter electrodes, theelectrodes of structure 210 (hereinafter array 210) will be described asfirst receiver electrodes, the electrodes of structure 220 (hereinafterarray 220) will be described as second transmitter electrodes, and theelectrode structure 320 will be described as a second receiver ordetector electrode. It will be appreciated, though, that the ability oftransducer 12 to produce signals indicative of position is not dependenton the direction of signal transmission through the transducer. Thus, anexcitation signal could be applied to electrode 320, making thatelectrode a transmitter electrode, and transducer outputs could beobtained from electrode array 310, making those electrodes receiverelectrodes. (Arrays 220 and 210 would then be first receiver and secondtransmitter arrays, respectively.) Further, it will be appreciated thatelectrode arrays 210 and 220 can be considered functionally as a singlearray structure capacitively coupling array 310 and electrode 320. Stillfurther, it will be appreciated that electrode 320 also can comprise anarray of electrodes, as shown in applicant's aforementioned copendingapplication Ser. No. 07/200,368. As depicted in FIG. 1, the electrodearray 310 and electrode 320 are disposed on support 30 so as to be inopposed spaced relationship to electrodes 210 and 220, respectively,during relative movement of the support members 20 and 30 along themeasurement axis.

In the embodiment shown, the first transmitter electrodes 310 are formedas a plurality of identical rectangular electrodes uniformly spacedadjacent to one another at predetermined intervals with a pitch Pt toform a rectangular envelope having a width Wt and a length TI extendingin the direction of the measurement axis X. As shown, successive seriesof a predetermined number greater than 1 (e.g., eight as shown) ofadjacent electrodes 312 are respectively connected to a correspondingseries of different excitation or input signals, and each series ofelectrodes constitutes a group spanning a distance defining onetransmitter wavelength Lt. The detector electrode 320 has a generallyelongate configuration in the form of a rectangle having a width Wr anda length RI disposed adjacent and parallel to the first transmitterelectrodes 310 and also extending in the direction of the measurementaxis X.

First receiver electrode array 210 comprises a plurality of individualrectangular electrodes 212 uniformly spaced adjacent to one another onthe support 20 in the direction of the measurement axis X. Secondtransmitter electrode array 220 likewise comprises a plurality ofindividual rectangular electrode elements 222 spaced adjacent to oneanother in the direction of measurement axis X. Each of the individualelectrodes 222 of the second transmitter electrode array 220 areelectrically coupled to corresponding individual electrodes 212 of thefirst receiver electrode array 210. First transmitter electrodes 312 andfirst receiver electrodes 212 are disposed on the supports 20 and 30,respectively, so that they maintain an opposed, spaced relationshipduring relative movement of the supports 20 and 30 along the measurementaxis. Likewise, the detector electrode 320 and second transmitterelectrodes 222 are disposed on supports 30 and 20, respectively, so asto maintain an opposed, spaced relationship during movement of thesupports 20 and 30 along the measurement axis.

As is more particularly described in the aforementioned copendingapplications, periodically varying excitation signals are applied to therespective groups of first transmitter electrodes 310 by signalgenerator 102. Signals from the first transmitter electrodes 312 arecapacitively coupled to the first receiver electrodes 212 andelectrically coupled to the second transmitter electrodes 222. Thesignals are then transmitted by capacitive coupling to the detectorelectrode 320, which produces an output signal that is detected by thesignal processor 104. The signal processor l04 then provides anindication of position by sensing the relationship between the signalstransmitted by first transmitter electrodes 312 and the signals receivedby detector electrode 320.

As was previously mentioned, in capacitance-type measuring devices ofthe type described, movement of the members 20 and 30 with respect toone another in directions other than the direction of measurement canhave significant effects on the accuracy of the measurement. Moreparticularly, there is shown in FIG. 2A a diagrammatic representation ofthe support members 20 and 30 disposed in a three-dimensional coordinatesystem. The coordinate system is defined by three axes, X, Y and Z,intersecting one another so that each axis is orthogonal to the other.The support members 20 and 30 are shown disposed in opposed relationshipto one another in planes which are parallel to one another and to theplane formed by the X and Y axis. Furthermore, the members 20 and 30 aredisposed to depict relative movement of the members in a direction alongthe measurement axis X.

As will be appreciated, measurement inaccuracies can be caused bymovement of the support members 20 and 30 and their associatedelectrodes with respect to one another in directions other than alongthe measurement axis. Such movements may occur as a result of themechanical tolerances of the parts which mount and guide the movement ofthe individual members 20 and 30 with respect to one another in thecaliper instrument. Such movements may also be characterized as fouralignment parameters, which can be identified as roll, tilt, yaw andlateral offset, as more particularly shown in FIGS. 2B-2E, respectively.More specifically, movement of either of the members 20 and 30 about theX axis, as depicted in FIG. 2B, is identified as roll; movement of thesupport members 20 and 30 about the Y-axis, as depicted in FIG. 2C, isidentified as tilt; movement of the support members 20 and 30 about theZ-axis, as depicted in FIG. 2D, is identified as yaw; and movement ofthe elements 20 and 30 with respect to one another along the Y-axis, asdepicted in FIG. 2E, is identified as lateral offset (Ld) ofdisplacement.

In a capacitance-type measuring device, each of the four alignmentparameters identified as yaw, roll, tilt and lateral offset Ld can havean effect on the accuracy of the measurement. Each of such movements arein direction other than the measurement direction and each willinfluence the accuracy of measurement depending upon the degree ofmovement. It is therefore apparent that capacitance-type measuringdevices manufactured with large mechanical tolerances in the structurethat maintains the relative positions of the support members 20 and 30and guides their relative movement, may lead to inaccurate measurements.Likewise, tightening of such tolerances will result in a more expensivemanufacturing construction, and therefore higher costs for the measuringdevice.

In capacitance-type measuring devices of the type described, the effectsof yaw, roll and lateral offset have been lessened by a variety oftechniques. More particularly, any sensitivity to yaw and lateral offsetcan be reduced by sufficient overlap between the electrode arrays on themembers 20 and 30. Furthermore, roll does not appear to influence thebalance between the various signals coupled between the firsttransmitter electrodes and detector electrode by way of the firstreceiver electrodes and second transmitter electrodes. However, theeffect of tilt produces significant measurement inaccuracies ininstruments with large mechanical tolerances.

By way of example, the effect of tilt on various designs can be moreclearly understood by reference to FIG. 2F, which shows the variation ofthe gap or spacing d between the elements 20 and 30 along the length ofa measurement axis to produce spacing h1 at one end and spacing h2 atthe opposite end. In order to illustrate the effect of the variablespacing caused by tilt, assume a nominal gap of 50 microns in FIG. 2Fbetween the two members 20 and 30 and a tilt of one micron per mm lengthof the member 30. For a 40 mm long support 30, the spacing h1 would be30 microns at one end and the spacing h2 would be 70 microns at theother end. In one example using an embodiment of the capacitance-typedevice described in U.S. Pat. No. 4,420,754 having a scale wavelength of2.54 mm, 4 transmitter phases, a 0.635 mm transmitter electrode pitch,and a 2.54 mm transmitter electrode wavelength, the resulting error forthe specified tilt will vary over the wavelength of the scale 20 andhave a peak-to-peak amplitude error of about 140 microns. This is anunacceptable error for a device like an electronic caliper with aresolution of 10 microns. In order to keep the error because of tiltdown to half the resolution (i.e., equal to 5 microns), the mechanicaltolerances in the guiding system controlling the location of the twoscales relative to each other would need to be improved to be not morethan 0.025 micron/mm, i.e., a 1 micron gap difference along the 40 mmextent of the scale 30.

In a second example using an embodiment of the capacitance-typemeasuring device described in applicant's aforementioned applicationSer. No. 07/200,580 having transmitter electrodes distributed over fivescale wavelengths and having a scale wavelength of 0.256 mm, eighttransmitter phases, a transmitter electrode pitch of 0.160 mm and atransmitter wavelength of 1.28 mm, the error for the given value of tiltwould again vary over the wavelength of the scale 20 with a peak-to-peakamplitude of about 2.5 microns. This is a much smaller error because ofthe shorter wavelength and the different sequence in which the phasesare connected. (The phase sequence is described in applicant'saforementioned application Ser. No. 07/200,368.) But there are new usesfor this capacitive sensor that require a resolution/accuracy of 1micron, with the potential need for further improvement to 0.1 micron.In those cases, the 2.5 micron error is unacceptable. In both examples,the length of the support member 30 will have only a marginal influenceon the error magnitude for a given tilt angle.

In accordance with the present invention, it has been determined thatthe effects of tilt can be significantly reduced by modifying thegeometry of the electrodes disposed on support member 30. In particular,by modifying the geometry of the end portions of transmitter electrodearray 310 or the detector electrode 320 such that the degree of couplingbetween the first transmitter electrode array and the detector electrodethrough the first receiver and second transmitter electrode arrayslinearly decreases relative to the measurement axis along the extent ofeach end portion, as described in more detail hereinafter, the errorcaused by tilt can be very considerably improved. Specifically, in thefirst example above, the error induced by tilt will be reduced by afactor of 50, to 2.8 micron. In the second example above, the error willbe reduced by a factor of 200, to the very small value of 0.01 micron.This allows capacitance-type devices to be constructed with lessmechanical tolerances and thereby enables the production of lessexpensive devices while maintaining an improved level of accuracy.

Referring particularly to FIG. 3, the same general caliper constructionas shown in FIG. 1 is depicted. In the embodiment of the inventionillustrated in FIG. 3, however, the detector electrode 320' isconstructed to have an elongate configuration in the form of a rectangleterminated at each end by a tapered portion. More specifically, detectorelectrode 320' generally includes three portions, identified as arectangular intermediate portion 322 and triangular terminal portions324A and 324B extending from opposite ends of portion 322 to form atapered electrode configuration in which the effective width of theelectrode tapers linearly relative to the measurement axis from the fullwidth Wr of the portion 322 to termination at each of the respectiveends 326A and 326B over a distance TA1.

Advantageously, the triangular portions 324A and 324B are configured asequilateral triangles which form a symmetrical taper about the electrodecenter axis f over the distance TA1. In this embodiment, the electrodes312 of the first transmitter electrode array 310 form an envelope ofgenerally rectangular configuration of length TI, and the plurality of Nelectrodes 312 forming an electrode group span a distance of onetransmitter wavelength Lt. The detector electrode 320' is configured asan elongate electrode having a length R1 (end to end) which is less thanthe length TI of the envelope of the first transmitter electrodes 310.Furthermore, the detector electrode 320' is advantageously constructedso that the length TA1 of the tapered portions 324A and 324B is equal toone transmitter wavelength Lt, and the length R1 of the detectorelectrode 320' is equal to an integer number of transmitter wavelengthsLt.

The second embodiment shown in FIG. 4 incorporates an alternativeconfiguration of the electrode geometries on support 30 in accordancewith the present invention. In this embodiment, the detector electrode320 has a rectangular configuration with a length R1. The envelope ofthe first transmitter electrode array 310', however, is configured tohave a length TI less than the length R1 of the detector electrode 320and equal to an integer number of transmitter wavelengths Lt. In thisembodiment, the first transmitter electrodes 312' form an envelope whichis defined by an intermediate portion 314 and two terminating portions316A and 316B similar in configuration to sections 322, 324A and 324B,respectively, in FIG. 3.

More particularly, envelope portion 314 is generally rectangular inshape and the two triangular portions 316A and 316B respectively extendfrom each end of portion 314 so that the envelope tapers linearly fromthe full width Wt of portion 314 to termination at each end 318A and318B over a distance TA1. Envelope portions 316A and 316B advantageouslyhave, as depicted, the shape of equilateral triangles which formsymmetrically tapered electrode envelope portions with respect to thecenter axis g extending the length of the elongate array 310'. As wasdescribed in connection with FIG. 3, it is preferable that the lengthTA1 of the tapered portions 316A and 316B be equal to one transmitterwavelength. However, improvements by a factor of 10 in certainconfigurations of capacitance-type measuring devices have been obtainedfor lengths TA1 equal to one-half the transmitter wavelength as shown inthe embodiment of FIG. 4.

As shown in FIG. 5, the end portions of an electrode structure in theform of an array of discrete electrodes, e.g., first transmitterelectrode array 310, can also provide a linear coupling gradient, i.e.,the aforementioned linear change in the degree of coupling between thefirst transmitter electrode array and the detector electrode, byprogressively tapering the length, relative to the measurement axis X,of the individual electrodes 312" comprising the end portions 316A and316B from the full length Le of portion 314 to termination at each ofthe respective ends 318A and 318B over the distance TA1. As shown, thetapered electrodes 312" of end portions 316A and 316B have the samepitch Pt (measured between the center lines of adjacent electrodes) asthe uniform length electrodes of intermediate portion 314. Preferably,the tapered electrodes are each tapered symmetrically with respect tothe electrode center line, that is each successive tapered electrode isreduced in length equally on both sides of the electrode centerline.

Although the present invention has been described with respect toembodiments having linear tapering geometries, it will be appreciatedthat if the individual electrodes of one of the electrode arrayscomprising transducer 12, e.g. first transmitter electrodes312/312'/312" or first receiver electrodes 212, have non-rectangularshapes e.g., triangular or sinusoidal as shown applicant'saforementioned copending application Ser. No. 07/200,368 incorporatedherein by reference, then a non-linear tapering geometry will berequired to achieve the linear coupling gradient between the firsttransmitter electrode array and detector electrode over the end portionsof the electrode array/electrode provided with the tapered geometry.

Further, although the preferred embodiments have been described abovewith respect to electrode structures having symmetrical taperinggeometries with lengths of taper equal to one transmitter wavelength,various degrees of improvement may still be obtained with other tapergeometries. By way of example, as specifically described above,improvements in accuracy for tilt movements can be obtained with taperlengths equal to one-half the transmitter wavelength in certaincapactive sensor configurations. Further, computer modelling indicatesthat taper lengths equal to any number of integer transmittingwavelengths will give about the same improvement in resultant errorcaused by tilt as tapering over one transmitter wavelength.

Still further, the use of tapered portions of other than symmetricalconfiguration are contemplated by the invention. For example, the taperconfiguration of the second receiver electrode arrays 320A and 320Bshown in FIG. 2A of applicant's copending application Ser. No.07/200,368 incorporated herein by reference provides significantimprovement over the non-tapered geometry shown in FIG. 1 herein.Indeed, if capacitive edge effects are not a factor, as long as theeffective width of the detector electrode or transmitter electrodeenvelope as a function of the X-coordinate (measurement direction)provides the above described linear coupling gradient, the geometricshape can vary almost in any way within the tapered portions of theelectrode/envelope. Three exemplary alternative embodiments of detectorelectrode 320' having non-symmetrical and irregularly shaped endportions 324A and 324B, which are equivalent to the symmetricaltriangular tapering of electrode 320' shown in FIG. 3, are shown inFIGS. 6-8, respectively. Preferably, as shown, end portions 324A and324D have complementary configurations, that is, the combined per phaseeffect of the two end portions on multiple phase signals transmittedthrough transducer 12 (between the first transmitter electrode array 310and detector electrode 320 through the first receiver electrode array210 and second transmitter electrode array 220) when there is no tiltbetween support members 20 and 30 is substantially the same as would beprovided by rectangular end portions of the same length.

Capacitive edge effects need not be considered if the regions of thetapered portions which have width dimensions approaching the nominaldimension of the spacing or gap between support members 20 and 30constitute a relatively small percentage of the tapered portions overone transmitter wavelength Lt. More specifically, referring to electrode320 of FIG. 7 as an example, the lengths Lma, Lmb of regions 327A and327B of electrode end portions 324A and 324B, which have respectivewidths Wma, Wmb approaching the size of the gap between support members20 and 30, preferably should each be less than approximately 5% of thedistance Lt.

Further, in the case of the embodiments shown in FIGS. 3 and 4, whereinthe cooperating electrodes 222 and 212, respectively, on support member20 are rectangular in shape, the tapered portions of detector electrode320' or the envelope of the first transmitter electrode array 310' canhave any location in the direction of the Y-axis (i.e., can be skewed)within the envelope of the cooperating electrodes, as long as theireffective widths as a function of the X-coordinate provide the linearcoupling gradient as described above.

While significant improvements may be obtained using the preferredembodiments, other forms of tapered geometries could be used dependentupon the degree of improvement desired or achievable with any givenconfiguration. Accordingly, the teachings of the inventions can be usedto provide improvements for a variety of capacitance-type transducers bymodifying one of the electrode configurations on support 30 to have atapered geometry For example, in the type of capacitive encoders whereeach support member contains only one array of electrodes, the presentinvention is applicable with the same effect on sensitivity to tilt. Inthis case, as is shown in FIG. 4, the tapering preferably is done on theenvelope of the electrodes on the shorter of the two support members,whether they are the transmitter or the receiver electrodes.

As can be seen from the above description, the use of the linearcoupling gradient configuration of the present invention in acapacitance-type measuring device provides an output of improvedaccuracy for variations in the tilt of the support members 20 and 30with respect to one another. The linear coupling gradient may be easilyimplemented by modifying the configuration of either the length or widthof individual electrodes in an elongate electrode array, or modifyingthe width of an elongate unitary electrode structure. In either case,the configuration provides an inexpensive improvement of the measurementaccuracy which allows the manufacture of the mechanical elements formingthe capacitance-type measuring device to less stringent tolerances. Thisreduces the overall cost and allows the measuring device to be used inapplications that would otherwise be prohibited by high cost orcomplexity.

It will be appreciated that many variations and modifications arepossible in light of the above teachings. It is therefore to beunderstood that the invention may be practiced otherwise than asspecifically described.

What is claimed is:
 1. A transducer comprising:first and second supportmembers, said support members being relatively displaceable with respectto each other, and at least one of said support members beingdisplaceable relative to a measurement axis; first electrode meansdisposed on said first support member, and second electrode meansdisposed on said second support member and capacitively coupled to saidfirst electrode means; said first and second electrode means each havingan elongate configuration extending in the direction of the measurementaxis, and one of said first and second electrode means having two endportions configured such that the degree of coupling between said firstand second electrode means decreases linearly relative to themeasurement axis along the extent of each end portion, and such that therespective directions of the decrease in the degree of coupling for saidtwo end members are opposite to each other, so that the effect of tiltbetween said first and second support members on a set of signalstransferred between said first and second electrodes is reduced.
 2. Thereducer of claim 1 wherein said one of said first and second electrodemeans comprises three portions: a first rectangularly-shaped portion;and second and third end portions extending from opposite ends of saidrectangular portion and constituting said two end portions; said secondand third end portions having complementary configurations such that thecombined per phase effect of said second and third end portions onmultiple phase signals transmitted between said first and secondelectrode means when there is no tilt between said first and secondsupport members is substantially the same as would be provided byrectangularly-shaped end portions of the same length.
 3. The transducerof claim 2 wherein said second and third end portions each have atriangular shape which is symmetrical with respect to a center line ofsaid one of said first and second electrode means.
 4. The transducer ofclaim 1 wherein said first electrode means has a shorter length in themeasurement axis direction than said second electrode means and saidfirst electrode means constitutes said one of said first and secondelectrode means.
 5. The transducer of claim 1 wherein said first andsecond electrode means respectively comprise arrays of discrete firstand second electrodes and said first electrodes are arranged in aperiodic fashion so as to constitute successive groups of N adjacentelectrodes, where N is an integer number greater than one, and saidgroups each have a wavelength Lt; and said two end portions each has alength in the measurement axis direction substantially equal to aninteger number of wavelengths Lt.
 6. The transducer of claim 5 whereinsaid first electrode means has a shorter length in the measurement axisthan direction said second electrode means and said first electrodemeans constitutes said one of said first and second electrode means. 7.The transducer of claim 6 wherein the length of said first electrodemeans is substantially equal to an integer number of wavelengths Lt. 8.The transducer of claim 1 wherein said first and second electrode meansrespectively comprise arrays of discrete electrodes, each of saidelectrodes has a length and a width relative to the measurement axis;and the lengths and/or widths of the electrodes comprising said two endportions progressively taper in size so as to obtain said linearlydecreasing degree of coupling.
 9. The transducer of claim 8 wherein saidelectrodes have a rectangular shape and said progressive taper islinear.
 10. The transducer of claim 8 wherein said first electrodemeans:constitutes said one of said first and second electrode means; hasa shorter length relative to the measurement axis than said secondelectrode means; and comprises three portions: a firstrectangularly-shaped portion, and second and third end portionsextending from opposite ends of said rectangularly-shaped portion andconstituting said two end portions.
 11. Transducer of claim 10 whereinthe electrodes of said first electrode means are arranged in a periodicfashion so as to constitute successive groups of N adjacent electrodes,where N is an integer number greater than one, and said groups each havea wavelength Lt; the length of said first electrode means issubstantially equal to a first integer number of wavelengths Lt; andsaid second and third end portions have a length substantially equal toa second integer number of wavelengths Lt, said second integer numberbeing less than said first integer number.
 12. A transducercomprising;first and second support members, said support members beingrelatively displaceable with respect to each other, and at least one ofsaid support members being displaceable relative to a measurement axis;first electrode means and second electrode means disposed on said firstsupport member; third electrode means disposed on said second supportmeans and capacitively coupling said first and second electrode meansfor transmission of signals between said first and second electrodemeans through said third electrode means; and said first and secondelectrode means each having an elongate configuration extending in thedirection of the measurement axis, and one of said first electrode andsaid second electrode means having two end portions configured such thatthe degree of coupling between said first and second electrode means,through said third electrode means, decreases linearly along the extentof each end portion relative to the measurement axis and such that therespective directions of the decrease in the degree of coupling for saidtwo end portions are opposite to each other, so that the effect of tiltbetween said first and second support members on a set of signalstransferred between said first and second electrode means, through saidthird electrode means, is reduced.
 13. The transducer of claim 12wherein said one of said first and second electrode means comprisesthree portions: a first rectangularly-shaped portion; and second andthird end portions extending from opposite ends of saidrectangularly-shaped portion and constituting said two end portions,said second and third end portions having complementary configurationssuch that the combined per phase effect of said second and third endportions on multiple phase signals transmitted between said first andsecond electrode means through said third electrode means when there isno tilt between said first and second support members is substantiallythe same as would be provided by rectangularly-shaped end portions ofthe same length.
 14. The transducer of claim 13 wherein said firstelectrode means comprises an array of discrete first electrodes arrangedin a periodic fashion so as to constitute successive groups of Nadjacent electrodes, where N is an integer number greater than one, andsaid groups each have a wavelength Lt; and said second electrode meansincludes said second and third end portions and has a length in themeasurement axis direction less than the length of said first electrodemeans and substantially equal to an integer number of wavelengths Lt.15. The transducer of claim 14 wherein said second and third, endportions each have a length in the measurement axis directionsubstantially equal to an integer number of wavelengths Lt.
 16. Thetransducer of claim 14 wherein said second and third end portions eachhave an effective width which progressively tapers relative to themeasurement axis so as to obtain said linearly decreasing degree ofcoupling.
 17. The transducer of claim 16 wherein the taper of therespective effective widths is linear.
 18. The transducer of claim 17wherein said second and third end portions each have a triangular shapewhich is symmetrical with respect to a center line of said secondelectrode means.
 19. The transducer of claim 13 wherein said firstelectrode means:comprises an array of discrete first electrodes arrangedin a periodic fashion so as to constitute successive groups of Nadjacent electrodes, where N is an integer number greater than one, andsaid groups each have a wavelength Lt; constitutes said one of saidfirst and second electrode means comprising three portions; and has alength in the measurement axis direction less than the length of saidsecond electrode means and substantially equal to an integer number ofwavelengths Lt.
 20. The transducer of claim 19 wherein said second andthird end portions each have a length in the measurement axis directionsubstantially equal to an integer number of wavelengths.
 21. Thetransducer of claim 20 wherein said second electrode means has arectangular shape.
 22. The transducer of claim 21 wherein said firstelectrodes of said first electrode means each have a length and a widthrelative to the measurement axis; and the lengths and/or the widths ofthe first electrodes of said second and third end portions progressivelytaper in size so as to obtain said linearly decreasing degree ofcoupling.
 23. The transducer of claim 22 wherein said third electrodemeans comprises an array of discrete third electrodes; said thirdelectrodes have a uniform rectangular shape; and said progressive taperis linear.
 24. A transducer comprising:first and second support memberswhich are relatively displaceable with respect to each other, and atleast one of which is displaceable relative to a measurement axis; anarray of first transmitter electrodes disposed on the first supportmember in alignment with the measurement axis; an array of firstreceiver electrodes disposed on the second support member in alignmentwith the measurement axis and such that different portions of the firstreceiver electrode array are capacitively coupled with the firsttransmitter electrode array in dependence on the relative positions ofsaid support members; an array of second transmitter electrodes disposedon the second support member in relative alignment with the firstreceiver electrode array and each of the second transmitter electrodesbeing electrically connected to a corresponding one of said firstreceiver electrodes; and detector electrode means disposed on the firstsupport member in relative alignment with the first transmitterelectrode array and capacitively coupled with the second transmitterelectrode array in dependence on the relative position of said supportmembers, one of said first transmitter electrode array and said detectorelectrode means being configured to have two end portions configuredsuch that the degree of coupling between said first transmitterelectrode array and said detector electrode means through said firstreceiver and second transmitter electrode arrays decreases linearlyalong the extent of each end portion relative to the measurement axisand such that the respective directions of the decrease in the degree ofcoupling for said two end members are opposite to each other, so thatthe effect of tilt between said first and second support members on aset of signals transferred between said first transmitter electrodearray and said detector electrode means is reduced.
 25. The transducerof claim 24 wherein a predetermined number of said first transmitterelectrodes defines a transmitter wavelength and said first transmitterelectrode array forms an envelope having a first rectangular portion andsecond and third end portions extending from opposite ends of saidrectangular portion, said envelope having a length in the measurementaxis direction less than the length of said detector electrode means andsubstantially equal to an integer number of transmitter wavelengths. 26.The transducer of claim 25 wherein said detector electrode means has arectangular configuration and each of said second and third end portionsof said envelope extends over a distance in the measurement axisdirection substantially equal to an integer number of transmitterwavelengths.
 27. The transducer of claim 26 wherein said firsttransmitter electrodes each have a length and a width relative to themeasurement axis; and the lengths and/or the widths of the firsttransmitter electrodes of said second and third end portionsprogressively taper in size so as to obtain said linearly decreasingdegree of coupling.
 28. The transducer of claim 27 wherein said firstreceiver electrodes have a uniform rectangular shape; and saidprogressive taper is linear.
 29. The transducer of claim 28 wherein onlythe lengths of the first transmitter electrodes of said second and thirdend portions progressively taper; and said second and third end portionseach have a triangular shape which is symmetrical with respect to acenter line of said first transmitter electrode array.
 30. Thetransducer of claim 24 wherein a predetermined number of said firsttransmitter electrodes defines a transmitter wavelength and saiddetector electrode means has an elongate configuration including a firstrectangular portion and second and third end portions extending fromopposite ends of said rectangular portion and each having an effectivewidth relative to the measurement axis which tapers so as to obtain saidlinearly decreasing degree of coupling; said detector electrode meanshaving a length in the measurement axis direction less than said firsttransmitter electrode array and substantially equal to an integer numberof transmitter wavelengths.
 31. A transducer comprising:first and secondsupport members disposed in opposed, spaced relation and movablerelative to one another along an axis of measurement; first electrodearray means disposed on said first support member along said measurementaxis, said first electrode array means forming a rectangular envelopeconfiguration having a predetermined length in the measurement axisdirection; second electrode array means disposed on said second supportmember along said measurement axis such that different portions of saidsecond electrode array means are capacitively coupled to said firstelectrode array means in response to relative movement of said first andsecond support members; and third electrode means disposed on said firstsupport member along said measurement axis such that said thirdelectrode means is capacitively coupled to different portions of saidsecond electrode array means in response to relative movement of saidfirst and second support members; said third electrode means having anelongate configuration with a length in the measurement axis directionless than the predetermined length of said first electrode array meansenvelope, and including a first rectangular portion and second and thirdend portions extending from opposite ends of said rectangular portionalong said measurement axis and each having an effective width whichprogressively tapers along said measurement axis such that the degree ofcoupling between said first electrode array means and said thirdelectrode means through said second electrode array means decreaseslinearly over said second and third end portions.
 32. The transducer ofclaim 31 wherein said second and third tapered end portions each has asymmetrical triangular shape.
 33. The transducer of claim 32 whereinsaid first electrode array means comprises first electrodes spaced fromone another along the measurement axis such that successive groups offirst electrodes are formed by equal numbers of adjacent firsttransmitter electrodes, said groups each having a wavelength Lt; and thelength of said third electrode means in the measurement axis directionless than the predetermined length of said first electrode array meansenvelope and substantially equal to an integer number of wavelengths Lt.34. The transducer of claim 33 wherein each of said second and thirdtapered end portions has a length in the measurement axis directionsubstantially equal to an integer number of wavelengths Lt.
 35. Thetransducer of claim 33 wherein each of said tapered end portions has alength in the measurement axis direction substantially equal to one-halfsaid wavelength.
 36. A transducer comprising:first and second supportmembers disposed in opposed, spaced relation and movable relative to oneanother along an axis of measurement; first electrode array meansdisposed on said first support member along said measurement axis, saidfirst electrode array means forming an envelope having an elongateconfiguration of predetermined length in the measurement axis directionand including a first rectangular portion and second and third endportions extending from opposite ends of said rectangular portion alongsaid measurement axis; second electrode array means disposed on saidsecond support member along said measurement axis such that differentportions of said second electrode array means are capacitively coupledto said first electrode array means in response to relative movement ofsaid first and second support members; and third electrode meansdisposed on said first support member along said measurement axis suchthat said third electrode means is capacitively coupled to differentportions of said second electrode array means in response to relativemovement of said first and second support members; said third electrodemeans having a rectangular configuration and said second and third endportions of said first electrode array means each being configured suchthat the degree of coupling between said first electrode array means andsaid third electrode means through said second electrode array meansdecreases linearly over said second and third end portions, and suchthat the respective directions of the decrease in the degree of couplingfor said second and third end portions are opposite to each other, sothat the effect of tilt between said first and third support members ona set of signals transferred between said first and third electrodemeans, through said second electrode means, is reduced.
 37. Thetransducer of claim 36 wherein said second and third end portions eachhas a symmetrical triangular shape.
 38. The transducer of claim 37wherein said first electrode array means comprises first electrodesspaced from one another along the measurement axis such that successivegroups of first electrodes are formed by equal numbers of adjacent firstelectronics, said groups each having a wavelength Lt, the length of saidfirst electrode array means envelope in the measurement axis directionbeing less than the length of said third electrode means andsubstantially equal to an integer number of wavelengths Lt.
 39. Thetransducer of claim 38 wherein each of said second and third endportions has a length in the measurement axis direction substantiallyequal to an integer number of wavelengths Lt.
 40. The transducer ofclaim 38 wherein each of said second and third end portions has a lengthin the measurement axis direction substantially equal to one-half saidwavelength Lt. .Iadd.
 41. A capacitance-type measurement transducercomprising:first and second support members, said support members beingrelatively displaceable with respect to each other, and at least one ofsaid support members being displaceable relative to a measurement axis;first and second electrode structures disposed on said first supportmember; a third electrode structure disposed on said second supportmember and capacitively coupling said first and second electrodestructures for signal transfer between said first and second electrodestructures through said third electrode structure; said first and secondelectrode structures each having an elongate configuration extending inthe direction of the measurement axis, and one of said first and secondelectrode structures having two end portions configured such that thedegree of coupling between said first and second electrode structures,through said third electrode structure, decrease relative to themeasurement axis along the extent of each end portion, and such that therespective directions of the decrease in the degree of coupling for saidtwo end members are opposite to each other, so that the effect of tiltbetween said first and second support members on signal transfer betweensaid first and second electrode structures is reduced. .Iaddend..Iadd.42. The transducer of claim 41 wherein said one of said first andsecond electrode structures having said two end portions includes anelongate middle portion intermediate said two end portions. .Iaddend..Iadd.43. The transducer of claim 41 wherein said first electrodestructure comprises an array of discrete first electrodes, a continuousmember constitutes said second electrode structure, and said thirdelectrode structure an array of discrete third electrode elements..Iaddend. .Iadd.44. The transducer of claim 41 further comprising signalgenerating means for applying at least one excitation signal to a firstone of said first and second electrode structures; and signal processingmeans for providing an indication of position in response to outputsignal(s) received from a second one of said first and second electrodestructures, said output signal(s) varying in accordance with acapacitance pattern which is created by said third electrode structureand said first or second electrode structure, and which changes as therelative positions of said support members change with respect to saidmeasurement axis. .Iaddend. .Iadd.45. The transducer of claim 42 whereinsaid middle portion has a generally rectangular shape, and said two endportions each has a generally triangular shape which is symmetrical withrespect to a center line of the corresponding electrode structure..Iaddend. .Iadd.46. The transducer of claim 43 wherein said firstelectrodes are arranged in a periodic fashion so as to constitutesuccessive groups of N adjacent electrodes, where N is an integer numbergreater than one, and said groups each has a wavelength Lt. .Iaddend..Iadd.47. The transducer of claim 46 wherein said first electrodestructure has a shorter length in the measurement axis direction thansaid third electrode structure and said first electrode structureconstitutes said one of said first and second electrode structures..Iaddend. .Iadd.48. The transducer of claim 47 wherein the length ofsaid first electrode structure is substantially equal to a first integernumber of wavelengths Lt. .Iaddend. .Iadd.49. The transducer of claim 47wherein each of said first electrodes has a length and a width relativeto the measurement axis; and the lengths and/or widths of the electrodescomprising said two end portions progressively taper in size so as toobtain said decreasing degree of coupling. .Iaddend. .Iadd.50. Thetransducer of claim 49 wherein said electrodes have a rectangular shapeand said progressive taper is linear. .Iaddend. .Iadd.51. The transducerof claim 47 wherein said first electrode structure further comprises arectangularly-shaped middle portion intermediate said two end portions..Iaddend. .Iadd.52. The transducer of claim 48 wherein said two endportions each has a length in the measurement axis directionsubstantially equal to a second integer number of wavelengths Lt, saidsecond integer number being less than said first integer number..Iaddend. .Iadd.53. The transducer of claim 46 wherein said secondelectrode structure includes said two end portions and has a length inthe measurement axis direction less than the length of said firstelectrode structure and substantially equal to an integer number ofwavelengths Lt. .Iaddend. .Iadd.54. The transducer of claim 53 whereinsaid two end portions each has a length in the measurement axisdirection substantially equal to an integer number of wavelengths Lt..Iaddend. .Iadd.55. The transducer of claim 53 wherein said two endportions each has a length in the measurement axis directionsubstantially equal to one-half of said wavelength Lt. .Iaddend..Iadd.56. The transducer of claim 53 wherein said two end portions eachhave an effective width which progressively tapers relative to themeasurement axis so as to obtain said decreasing degree of coupling..Iaddend. .Iadd.57. The transducer of claim 56 wherein the taper of therespective effective widths is linear. .Iaddend. .Iadd.58. Thetransducer of claim 57 wherein said end portions each has a triangularshape which is symmetrical with respect to a center line of said thirdelectrode structure. .Iaddend. .Iadd.59. The transducer of claim 47wherein said first electrode structure has a length in the measurementaxis direction less than the length of said second electrode structureand substantially equal to an integer number of wavelengths Lt..Iaddend. .Iadd.60. The transducer of claim 59 wherein said two endportions each has a length in the measurement axis directionsubstantially equal to an integer number of wavelengths Lt. .Iaddend..Iadd.61. The transducer of claim 59 wherein said two end portions eachhas a length in the measurement axis direction substantially equal toone-half of said wavelength Lt. .Iaddend. .Iadd.62. The transducer ofclaim 60 wherein said second electrode structure has a rectangularshape. .Iaddend. .Iadd.63. The transducer of claim 62 wherein each ofsaid first electrodes has a length and a width relative to themeasurement axis; and the lengths and/or widths of the electrodescomprising said two end portions progressively taper in size so as toobtain said decreasing degree of coupling. .Iaddend. .Iadd.64. Thetransducer of claim 63 wherein said progressive taper is linear, andsaid third electrode structure comprises rectangular electrode portions..Iaddend. .Iadd.65. The transducer of claim 44 wherein:said firstelectrode structure constitutes a first transmitter electrode structure;said third electrode structure comprises an array of first receiverelectrodes disposed on the second support member in alignment with themeasurement axis and such that different portions of the first receiverelectrode array are capacitively coupled with the first electrodestructure in dependence on the relative positions of said supportmembers, and an array of second transmitter electrodes disposed on thesecond support member in relative alignment with the first receiverelectrode array, with each of the second transmitter electrodes beingelectrically connected to a corresponding one of said first receiverelectrodes; and said second electrode structure constitutes a detectorelectrode and is disposed on the first support member in relativealignment with the first transmitter electrode structure and so as to becapacitively coupled with the second transmitter electrode array independence on the relative position of said support members. .Iadd.66.The transducer of claim 44 wherein:said first electrode structureconstitutes detector-electrode structure; said third electrode structurecomprises an array of first receiver electrodes disposed on the secondsupport member in alignment with the measurement axis and such thatdifferent portions of the first receiver electrode array arecapacitively coupled with the second electrode structure in dependenceon the relative positions of said support members, and an array ofsecond transmitter electrodes disposed on the second support member inrelative alignment with the first receiver electrode array, with each ofthe second transmitter electrodes being electrically connected to acorresponding one of said first receiver electrodes; and said secondelectrode structure constitutes a first transmitter electrode structureand is disposed on the first support member in relative alignment withthe detector electrode structure and so as to be capacitively coupledwith the first receiver electrode array in dependence on the relativeposition of said support members. .Iadd.67. The transducer of claim 41wherein the degree of coupling between said first and second electrodestructures decreased linearly along the extent of each end portionrelative to the measurement axis, and said two end portions havecomplementary configurations such that the combined per phase effect ofsaid end portions on multiple phase signals transmitted between saidfirst and second electrode structures through said third electrodestructure when there is no tilt between said first and second supportmembers is substantially the same as would be provided byrectangularly-shaped end portions of the same length. .Iaddend..Iadd.68. A capacitance-type measurement transducer comprising: firstand second support members, said support members being relativelydisplaceable with respect to each other, and at least one of saidsupport members being displaceable relative to a measurement axis; afirst electrode structure comprising an array of discrete firstelectrodes disposed on said first support member, and a second electrodestructure comprising an array of discrete second electrodes disposed onsaid second support member and capacitively coupled to said firstelectrode structure; said first and second electrode structures eachhaving an elongate configuration extending in the direction of themeasurement axis, and one of said first and second electrode structureshaving two end portions configured such that the degree of couplingbetween said first and second electrode structures decreases relative tothe measurement axis along the extent of each end portion, and such thatthe respective directions of the decrease in the degree of coupling forsaid two end portions are opposite to each other, so that the effect oftilt between said first and second support members on signal transferbetween said first and second electrode structures is reduced. .Iadd.69.A capacitance-type measurement transducer comprising:first and secondsupport members, said support members being relatively displaceable withrespect to each other, and at least one of said support members beingdisplaceable relative to a measurement axis; a first electrode structuredisposed on said first support member, and a second electrode structuredisposed on said second support member and capacitively coupled to saidfirst electrode structure; said first and second electrode structureseach having an elongate configuration extending in the direction of themeasurement axis, and one of said first and second electrode structureshaving two end portions separated by an elongate middle portion, saidend portions being configured such that the degree of coupling betweensaid first and second electrode structures decreases relative to themeasurement axis along the extent of each end portion, and such that therespective directions of the decrease in the degree of coupling for saidtwo end portions are opposite to each other, so that the effect of tiltbetween said first and second support members on signal transfer betweensaid first and second electrode structures is reduced and said middleportion being configured such that the degree of coupling between saidfirst and second electrode structures does not progressively decreaseover the extend of the middle portion. .Iadd.70. The transducer of claim69 wherein said first electrode structure comprises an array of discretefirst electrodes, and said second electrode structure comprises an arrayof discrete second electrodes. .Iaddend.